The goal of this research is to obtain metrical information from continuous wave and pulsed EPR spectroscopy of interacting spin systems in biological molecules. EPR transition energies, transition probabilities, and relaxation times will be calculated for spin systems of interest. These calculations will be calibrated with discrete metal complexes containing paramagnetic metals and nitroxyl spin labels. Exchange and dipolar contributions to the spin-spin interaction will be measured. The limits of applicability of various approaches to measuring spin-spin distances will be determined. These results will be applied to the nickel metalloenzyme urease, the copper-containing plastocyanin, and to CaATPase.